Among those pigments formed by coating a flaky substrate with a metal oxide having a high refractive index are titanium oxide-coated mica pigment and iron oxide-coated mica pigment. These pigments are characterized by the optical interference they produce. For the interference color to have a high saturation, it is necessary that the surface of flaky substrate particles be coated with a uniform, dense layer of fine, uniform particles of metal oxide. This metal oxide layer produces an interference color when its optical thickness is properly controlled.
Pigments producing an interference color are attracting attention because of their aesthetic nature, and they are now in general use for paint, plastics, ink, and cosmetics. However, a metal oxide-coated mica pigment is limited in its application due to its poor hiding power inherent in its optical characteristics. This poor hiding power leads to dichroism which has an adverse effect on the color. (Dichroism is due to interference of light reflected at a certain angle by a white substrate and a paint of poor hiding power applied thereon.)
Coloring pigments are required to have a high hiding power and superior chromaticity as well as good light resistance and weather resistance if they are to be added to automotive finishing paints, architectural paints, printing inks, and plastic products for outdoor use. In the case of automotive finishing paints, the requirements are met by graphite flaky particles coated with a metal oxide. Graphite flaky particles have a high hiding power and adsorb transmitting interference colors owing to their black smooth surface, and the metal oxide coating produces interference colors.
Titanium dioxide-coated graphite pigments are known as disclosed in Japanese Patent Publication No. 3824/1974 and Japanese Patent Laid open No. 87422/1977. They do not necessarily meet requirements for various color effects. According to the first disclosure, the pigment is prepared by suspending an untreated graphite substrate in water with stirring and then coating it with titanium dioxide. A disadvantage of this process is that the graphite substrate is not thoroughly dispersed in water and hence is not uniformly coated with titanium dioxide. The resulting pigment, therefore, does not produce a satisfactory interference color. According to the second disclosure, the pigment is prepared by suspending natural flaky graphite (as the substrate) in a titanyl sulfate solution and hydrolyzing it with heating, thereby coating the substrate with titanium dioxide. A disadvantage of this process is that the titanium dioxide coating is not so dense as required and involves difficulties in controlling its thickness and particle size.
The pigments disclosed in Japanese Patent Publication Nos. 3824/1974 and 16531/1974 and Japanese Patent Laid-open No. 87422/1974. Thus do not necessarily produce good interference colors, nor do they exhibit satisfactory light resistance and weather resistance.